Because of its relatively low cost, linear tape is commonly used as a medium for storing large amounts of digital data for archival purposes. For example, disk-based memory is often archived on linear data storage tape.
Data is formatted on linear tapes in a plurality of tracks that extend longitudinally along the tape. A tape read/write head is moveable laterally across the tape to read or write different tracks. In many cases, multiple tracks can be written or read at the same time by using a tape head with multiple read/write elements.
When reading or writing a linear data storage tape, accurate lateral positioning of the tape head is very important. To achieve such accuracy, servo bands are prewritten to the tape. The servo bands are detected by the tape head during reading and writing to determine the exact lateral position of the tape head relative to the linear tape.
FIG. 1 illustrates, conceptually, the use of servo bands. FIG. 1 shows a segment of a linear tape 10 that extends in a longitudinal direction x, and that has a lateral dimension y. The tape includes a plurality of servo bands 12. In the simplified example of FIG. 1 there are three servo bands. The servo bands are written to the tape during a preparatory "formatting" process, prior to actual use of the tape for data storage. The servo bands are spaced laterally from each other by a specified distance.
Data tracks 14 are located between the servo bands. The lateral positions of the data tracks are specified relative to the servo bands.
When reading or writing tape 10, a tape head senses the servo bands with servo read elements and positions itself precisely relative to the servo bands. Within the tape head, data read/write elements are spaced relative to the servo read elements so that the data read/write elements will be positioned over data tracks 14 when the servo read elements are positioned accurately over the corresponding servo bands.
In an actual embodiment, a linear tape might have more than three servo bands, with discrete tracks defined within each servo band. Many tape heads are configured to span two adjacent servo bands at any given time and to read or write only the data tracks between those servo bands. To read or write other data tracks, such a tape head is repositioned to span two different servo bands.
There are different ways to derive lateral position information from a servo band. One common way is to divide a servo band into two half tracks, which are recorded with different information (such as two distinct frequencies or bursts occurring at distinct times). A single servo head straddles the boundary between the half tracks, and position information is obtained by comparing the amplitude or phase responses of the signals generated from the respective half tracks.
A different approach has been described in Albrecht, et al., Time-Based, Track-Following Servos for Linear Tape Drives, Data Storage Magazine, 1997 (p. 41), which is hereby incorporated by reference. This approach uses timing-based, continuously-variable servo bands which are read by narrow servo heads.
FIG. 2 shows an example of a continuously-variable, timing-based servo pattern, along with a signal generated by a servo read element positioned over the servo pattern. The pattern consists of alternating magnetic transitions at two different azimuthal slopes. Relative timing of pulses generated by the read element depends on the lateral position of the head.
More specifically, the servo band illustrated in FIG. 2 has a series of magnetic transitions 20 and 22 referred to as "stripes" that are recorded on the tape with alternate azimuthal slopes. Every other stripe 20 has a positive, non-zero slope or azimuth, while the intervening stripes 22 have negative slopes or azimuths. Note that such azimuths are specified relative to the lateral or y tape direction (across the width of the tape).
FIG. 2 shows the path and width of a servo read element, indicated by reference numeral 24, that is designed for use with this tape. The servo head reads a lateral width that is significantly less than the full lateral widths of the stripes themselves. The signal generated by the servo head is represented by trace 26, illustrated directly below the illustrated magnetic transition stripes. When the servo head encounters a stripe, it generates a positive pulse. When the servo head leaves the stripe, it generates a negative pulse.
Lateral position information can be derived by comparing the distances between pulses. For example, a first distance A can be defined as the distance from a positive stripe to the next negative stripe, while a second distance B can be defined as the distance from a negative stripe to the next positive stripe. When the servo head is centered over the servo band, A will be equal to B: consecutive pulses will occur at equal intervals.
In actual implementation, alternating "bursts" of stripes are used, with a burst being defined as one or more individual magnetic transition stripes.
FIG. 3 shows an example of a servo band layout utilizing alternating bursts of magnetic transition stripes. Each burst has an opposite azimuthal slope from the previous burst. The servo pattern includes repeating frames. Each frame has a first subframe A and a second subframe B. Each subframe has a pair of bursts, with the bursts of each frame having different azimuthal slopes. Subframe A has a first burst 38 with five equally-spaced stripes having a positive azimuthal slope. Subframe A has a second burst 40 with five equally-spaced stripes having negative azimuthal slopes. Subframe B has similar bursts 42 and 44, except each of these bursts has four stripes.
It is preferable to record servo bands on a tape prior to its actual use for storing data. In order to ensure precise spacing of servo bands such as shown in FIGS. 2 and 3, it is desirable to use a patterned write head, configured to simultaneously write corresponding stripes of multiple servo bands.
FIG. 4 shows an example of patterned servo write head 46 for writing multiple servo bands on a single linear tape. The head's write pattern is illustrated relative to an underlying linear tape 47 that has four servo bands 48. At lateral positions corresponding to each servo band, the write head pattern includes pairs of write elements or gaps. One element 50 of each pair is configured to produce a magnetic transition stripe having a positive slope. Another element 52 of each pair is configured to produce a magnetic transition stripe having a negative slope. Using this configuration, a single current pulse to the head writes stripes simultaneously to all of the servo bands. Such pulses are repeated to produce stripes in the desired pattern, with the desired spacing.
After writing the servo bands, it is highly desirable to verify that the stripes have been correctly written to the tape. Defective magnetic transitions might result from write head clogging, from interference with debris, and from servo write head wearing. In addition, defects such as scratches in the tape itself can cause un-writable regions that result in signal dropouts, noise, and other errors. Tape defects in the form of scratches are particularly troublesome because they can cause offsets in detected position signals.
There are several different ways that servo band verification might be accomplished. One way would be to read the servo band stripes with a plurality of conventional, narrow, data read elements positioned across the width of the tape. However, this would require an unwieldy number of such read elements in order to cover the width of each servo band.
Another way would be to use a read head configured similarly to the servo write head. Such a read head would have relatively wide read elements oriented with azimuths matching the azimuths of the written stripes themselves.
FIG. 5 shows an example of a servo band having a longitudinal scratch that is approximately 10% of the width of the entire servo band. Shown in FIG. 5 are servo stripes 60 with alternating positive and negative azimuths. FIG. 5 also shows a servo verify head 61 with a pair of wide read elements 62 and 64 (shown by dashed line to distinguish them from the servo band stripes) positioned at positive and negative azimuths to match the azimuths of the servo band stripes 60. Read element 62 is used to verify stripes having a positive azimuth. Read element 64 is used to verify stripes having a negative azimuth. A region 68, delineated by a dashed rectangle, indicates a defective area on the tape which does not record magnetic transitions. Thus, portions of stripes 60 are missing when the pulses overly area 68.
Waveform 66 illustrates a signal produced by read element 62. When read element 62 encounters a matching stripe, it generates a positive pulse and then a negative pulse. When the stripe has a 10% defect, a pulse is generated having an amplitude that is approximately 90% of its normal amplitude. By monitoring the amplitudes of these pulses and noting those having low amplitudes, it is theoretically possible to detect defects. In practice, however, a 10% signal variation is not enough for reliable detection. Thus, although a scratch having a width of 10% of the servo band is serious enough to impair servo positioning capability, it is perhaps not wide enough for the scheme of FIG. 5 to reliably detect.
The inventors have developed a different way to verify servo bands of the type described above. The invention allows full verification of servo bands while requiring only a single, wide, read element for each servo band.